RockerB,
This won't help you with your immediate problem but I use this book for what you are describing.
http://www.pipefitters.com/ Pipe Trades Pocket Manual is the book that I'm referring to. Click books and look for A104. It gives the cut back ordinates and the center to end dimension for cutting down 90 elbows and it has lots of other prefigured charts that are very helpful. You don't have to go through the formula process the book gives you the numbers and they work very well.
This is my favorite pipefitting book.